1. Field of the Invention
The present invention relates to image forming apparatus such as facsimile and copying machines, and, more particularly, to an image forming electrophotographic apparatus and process.
2. Description of the Related Art
The conventional image forming apparatus is shown in FIG. 10 of the accompanying drawings to include a photosensitive drum 1. A charging device 2, an exposure device 3, a developing device 4, a transfer device 6, and a cleaning device 8 are arranged around the outer circumferential surface of the photosensitive drum 1. Of these components, the photosensitive drum 1, the charging device 2, the developing device 4, and the cleaning device 8 are integrally supported by side frames (not shown) to provide the assembled process unit 100.
In operation, the photosensitive drum 1 is rotated in the clockwise direction, the charging device 2 charges the surface of the photosensitive drum 1 to a predetermined potential (e.g., -600 V) and the exposure device 3 is controlled in response to an image data of an image to be recorded.
The developing device 4 comprises a toner hopper 41, a toner pack 42, a feed roller 43, a developing roller 44, a developing blade 45, a support rod 46, a leaf spring 47, a support 48, and a reinforcing plate 49.
The toner hopper 41 is a hollow container, having partially open upper surfaces, for storing toner (not shown) therein. The toner pack 42 is mounted on the upper open portion of the toner hopper 41. The toner pack 42 is filled with the toner and is provided with an opening initially sealed with a removable seal sheet (not shown). When the seal sheet is removed and the toner pack 42 is mounted on the toner hopper 41, the toner empties into the toner hopper 41.
The feed roller 43 is arranged at an opening on the side surface of the toner hopper 41 such that it is partly located in the toner hopper 41. The developing roller 44 lightly contacts both the photosensitive drum 1 and the feed roller 43. The feed roller 43 is rotated in the counterclockwise direction by a rotary drive mechanism (not shown). The developing roller 44 is also rotated in the counterclockwise direction. The feed roller 43 carries the toner stored in the toner hopper 41 and supplies it to the developing roller 44. The developing roller 44 carries the toner given by the feed roller 43 and causes it to contact the surface of the photosensitive drum 1.
The cylindrical support rod 46 is arranged parallel to and above the developing roller 44 to support the developing blade 45 in contact with the developing roller 44. The leaf spring 47, which is fixed to the support 48, urges the support rod 46 toward the developing roller 44 with a predetermined force F. Thus, the developing blade 45 is urged against the developing roller 44. The support 48 is fixed to a side wall of the toner hopper 41.
The transfer device 6 includes a transfer roller 61 rotating in a counterclockwise direction in contact with the photosensitive drum 1. The transfer device 6 further includes a positive direct-current power source 64 and a negative direct-current power source 65. The transfer roller 61 is alternatively coupled to one of the positive direct-current power source 64 and the negative direct-current power source 65.
The cleaning device 8 includes a cleaning blade 82 contacting the photosensitive drum 1 for cleaning toner remaining on the photosensitive drum 1 after the transferring operation.
In the facsimile apparatus having the structure as described above, an image is printed in the following manner.
First, the charging device 2 charges the photosensitive surface of the photosensitive drum 1 to a predetermined potential (e.g., -600 V). Subsequently, the exposure device 3 exposes the charged photosensitive surface of the drum 1 in accordance with an image to be printed, thereby forming an electrostatic latent image. Then, the developing device 4 develops the electrostatic latent image formed on the photosensitive surface of the drum 1.
In the developing device 4, the feed roller 43 carries the toner to the developing roller 44. The developing roller 44 carries and conveys the toner. When the developing roller 34 rotates, toner carried on the developing roller 34 receives friction between the developing roller 44 and developing blade 45. As a result, the toner is charged by friction. An acrylic resin layer, which tends to be charged in positive polarity, is provided at the surface of the developing roller 44. The resin layer is charged with positive polarity by friction with the toner. Accordingly, the toner is charged with negative polarity due to the polarization effect caused by the resin layer.
A developing bias, for example -200 V, having the same polarity as that of the potential charged on the photosensitive drum 1, is applied to the developing roller 44. The toner selectively attaches to the photosensitive drum 1 due to the presence of the electric field among the electrostatic latent image, the developing bias, and the charge of toner. More particularly, the toner does not attach to the non-exposed portion of the photosensitive drum 1 since the potential at this portion of the photosensitive drum 1 is more negative than that of the developing roller 44 transporting the toner. The toner attaches to the exposed portion of the photosensitive drum 1 since the potential at this portion of the photosensitive drum 1 is less negative than that of the developing roller 44 transporting the toner. In this manner, a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive drum 1. This toner image is transferred to the paper P by a positive voltage of the transfer roller 61, which is supplied by the positive direct-current power source 64.
After the printed sheet P is separated from the photosensitive surface of the photosensitive drum 1, the toner which is not transferred, and remains on the surface of the photosensitive drum 1, is removed by the cleaning device 8.
While the paper P is conveyed, a paper jam sometimes occurs. When the paper jam occurs, the photosensitive drum 1, the feed roller 43, the developing roller 44, and the transfer roller 61 continue to be rotated until the user removes the jammed paper from the apparatus. During this period, the transfer roller 61 remains supplied with the positive voltage from the positive direct-current power source 64. Therefore, the toner adhering on the photosensitive drum 1 having negative charge is transferred to the transfer roller 61 supplied with the positive voltage.
In this state, if the user removes the jammed paper from the apparatus and the apparatus performs a next recording operation, the toner transferred to and adhering to the transfer roller 61, transfers and adheres to the surface of the paper opposite to a recorded surface since the transfer roller 61 urges the paper toward the photosensitive drum 1, and therefore contacts the paper. As a result, the surface of the paper becomes dirty due to the toner from the transfer roller 61.
FIG. 11 shows a conventional sequence for solving the above deficiency. T1 is an instant of time following a predetermined period of time after the jammed paper is removed from the apparatus and a cover of the apparatus (not shown) is closed. T2 is an instant of time when a next recording operation starts. A period from the time T1 to the time T2 is referred to as a period TJ.
During the period TJ, the photosensitive drum 1 and the transfer roller 61 rotates. The charging device 2 charges the photosensitive drum 1 during the period TJ at -600 V. Furthermore, the developing roller 44 is supplied with the predetermined voltage, e.g., -200 V.
Furthermore, the negative voltage (e.g., -1350 V) from the negative direct-current power source 65 has been supplied to the transfer roller 61 during the period TJ. Thus, the toner including negative charge adhering to the transfer roller 61 during the paper jam, is repelled to the photosensitive drum 1 rotating in contact with the transfer roller 61 during the period TJ since the potential of the transfer roller 61 (-1350 V) is more negative than that of the photosensitive drum 1 (-600 V). During the period TJ, rotation of the developing roller 44 is stopped so that the developing roller 44 does not supply toner from the developing device 4 to the photosensitive drum 1.
Most of the toner on the drum 1 at this time is negatively charged. However, there is some toner with a positive charge more than negative charge adhering on the photosensitive drum 1 while the paper jam occurs. The ratio of the toner with a positive charge more than negative charge is several percent. This results from the toner receiving insufficient friction between the developing roller 44 and the developing blade 45. For this reason, toner having a positive charge on the photosensitive drum 1 is not repelled against the transfer roller 61 supplied with negative voltage. As a result, the toner having positive charge is attracted to the transfer roller 61 during the period TJ and causes the surface of the paper opposite to the recorded surface to be dirty during the next recording operation.